ON HYDRAULICS AS A BRANCH OF ENGINEERING. 169 



in the ratio of 24 to 10, by a certain form of tube, — a circum- 

 stance of which he supposes the Romans were well aware, as 

 appears from their restricting the length of the pipes of con- 

 veyance from the pubHc reservoirs to fifty feet ; but it was not 

 perceived that the law might be equally evaded by applying a 

 conical frustrum to the extremity of the tube. 



M. Venturi then examines the causes of eddies in rivers; 

 whence he deduces from his experiments on tubes with en- 

 larged parts, that every eddy destroys part of the moving force 

 of the current of the river, of which the course is permanent 

 and the sections of the bed unequal, the water continues more 

 elevated than it would have done if the whole river had been 

 equally contracted to the dimensions of its smallest section, — a 

 consequence extremely important in the theory of rivers, as the 

 retardation experienced by the water in rivers is not only due 

 to the friction over the beds, but to eddies produced from the 

 irregularities in the bed, and the flexures or windings of its 

 course : a part of the current is thus employed to restore an 

 equilibrium of motion, which the current itself continually de- 

 ranges. As respects the contracted vein, it had been pretended 

 by the Marquis de Lorgna* that the contracted vein was no- 

 thing else but a continuation of the Newtonian cataract ; and 

 that the celerity of the fluid issuing from an orifice in a thin 

 plate is much less than that of a body which falls from the 

 height of the charge. But Venturi proved that the contraction 

 of the vein is incomparably greater than can be produced by 

 the acceleration of gravity, even in descending streams, the 

 contraction of the stream being 0*64, and the velocity nearly 

 the same as that of a heavy body which may have fallen through 

 the height of the charge. These experimental principles, which 

 are in accordance with the results of Bossut, Michelotti and 

 Poleni, are strictly true in all cases where the orifice is small in 

 proportion to the section of the reservoir, and when that orifice 

 is made in a thin plate, and the internal afflux of the filaments 

 is made in an uniform manner round the orifice itself. Venturi 

 then shows the form and contraction of the fluid vein by in- 

 creased charges. His experiments with the cone are curious ; 

 and it would have been greatly to be regretted that he had 

 stopped short in his investigations, but for the more extensive 

 researches of Bidone and Lesbros. M. Hachette, in opposition 

 to the theory of Venturi, assigns, as a cause of the increase by 

 additional tubes, the adhesion of the fluid to the sides of the . 

 tubes arising from capillary attraction. 



* Memorie della Societa Ifaliana, vol. iv. 



